Electronic display elements, such as vacuum fluorescent tubes, have a predetermined acceptable range of filament voltages (i.e. typically plus or minus 10%). This voltage range is extremely critical for vacuum fluorescent displays. If the filament voltage is too high, vacuum fluorescent tubes have a lower life expectancy and oftentimes even result in damage to the displays. If the filament voltage is too low, the brightness of the display is lowered.
This limitation makes it difficult to supply filament voltages straight from a 50/60 Hz step-down transformer, especially for industrial applications where input supply voltages to such a transformer may vary 15% or more.
One current approach is to run the voltage straight from the 50/60 Hz step-down transformer secondary winding with the correct filament voltage. For industrial applications where the input supply voltage may vary plus or minus 15or more, the straight secondary approach will not keep the filament voltage within specification. If a filament over-voltage condition occurs for a significant period of time, the display life of the filament is greatly reduced.
Another approach for generating an AC filament voltage is to rectify and regulate the AC transformer secondary voltage to DC and then chop it at a 50% duty cycle back to AC at the necessary filament voltage. This approach addresses the problem of the filament voltage wandering outside its voltage specification. However, this approach has at least two drawbacks. For large vacuum fluorescent tubes with filament currents exceeding 0.5 amps, the size and number of components is increased. Also, the large amount of power dissipation is wasted on the rectifiers and regulators, thereby making this approach an expensive and undesirable alternative.